mailbox: k3-sec-proxy: Extend valid thread IDs

AM64x uses a different thread mapping that existing K3 SoCs, so update
the valid thread ID list to include those used for AM64x.

Also remove the comment identifying the purpose of each thread ID. The
purpose of the thread ID is specified when describing the threads in the
device tree and the same ID can mean different things on different SoCs,
so the comment is not useful.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>